I have found that I,(170lbs) require about 2-3 shells @ 2-3 tbsps of dried kava for the full krunk - while my similar sized friend needs about twice as much as me.
There are too many other factors that affect the bioavailability of kava besides body mass, such as set&setting, how much food you have in your stomach, your expectations and of course the type of blend of kava you are using.
It can take some time to dial in your dose.